The invention relates to ion sources and more particularly to multicusp ion sources.
In many applications, an ion source that can provide low longitudinal (axial) energy spread is required. Ion Projection Lithography (IPL) aims at projecting sub-0.1 .mu.m patterns from a stencil mask onto a wafer substrate. In order to keep the chromatic aberrations below 25 nm, an ion source which delivers a beam with an energy spread of less that 3 eV is required. In the production of radioactive ion beams for nuclear physics experiments, an ion source with axial energy spread less than 1 eV is needed to perform isobaric separation with a magnetic deflection spectrometer. In low energy (&lt;100 eV) ion beam deposition processes, very low energy spread is required in order to separate and focus the ions properly. Low energy (&lt;500 eV) mass spectrometers for analyzing nuclear and chemical waste need an ion source that has low longitudinal energy spread to achieve good mass resolution.
An ion source is a plasma generator from which beams of ions can be extracted. A multicusp ion source has an arrangement of magnets that form magnetic cusp fields to contain the plasma. The plasma generating source is surrounded by columns of permanent magnets. The magnets are placed around the cylindrical side wall as well as an end flange. In most cases an extraction system is placed at an open end. Such magnet placement results in an asymmetric distribution of the plasma potential inside the source which produces an axial or longitudinal energy spread.
U.S. Pat. No. 4,793,961 issued Dec. 27, 1988 to Ehlers et al. describes a multicusp ion source.
A multicusp ion source is needed which can provide a low longitudinal or axial energy spread for many applications. This is especially true when ion beams must be transported, manipulated, analyzed and applied in very low energy applications.
The ions and electrons in a plasma are charged particles in motion and experience an interaction with a magnetic field. The ions and electrons move in orbits around the magnetic field lines and, apart from collisions with other plasma particles, act as though they are tied to the field lines. The behavior of a plasma in a magnetic field can be profoundly different from a plasma in the absence of a magnetic field.
The change in direction of motion of ions and electrons in the presence of a magnetic field provides a means of confining the plasma, at least in the direction transverse to the field. Plasma loss along the field can be reduced by increasing the field strength at the ends of the confinement region. The multicusp ion source uses this principle to successfully generate and confine the plasma.
Multicusp fields have three important effects on low-pressure plasma discharges. High energy electrons can be efficiently confined. These electrons can be the ionization source for a discharge. Significant improvements can be obtained in the confinement of the bulk plasma in a discharge. Significant improvements in radial plasma density and potential uniformity can be achieved.
Plasma can be generated in a multicusp ion source by dc discharge or RF induction discharge. The surface magnetic field generated by rows of permanent magnets, typically of samarium-cobalt, can confine the primary ionizing electrons very efficiently. As a result, the ionization efficiency of this type of plasma generator is high.
In the case of dc discharge, the primary ionizing electrons are normally emitted from hot tungsten-filament cathodes. The source chamber walls form the anode for the discharge. There are three main components in the source: the cathode, the anode, and the first or plasma electrode. Two dc power supplies are needed to produce plasma by means of a dc filament discharge. One is for filament heating (the heater power supply) and the other is for the discharge (the discharge power supply). The discharge or arc voltage usually ranges from 40 to 100 V.
There are two ways in which a low pressure gas can be excited by RF voltages: (1) a discharge between two parallel plates across which an alternating potential is applied (capacitively coupled discharge), and (2) a discharge generated by an induction coil (inductively coupled discharge). Most RF-driven ion sources are operated with the second type of discharge. A few hundred watts of RF power is typically required to establish a suitable discharge. The RF frequency can vary from a megahertz to tens of megahertz.
In the plasma source, the ions are generated in a discharge chamber. From that point of generation they drift until a fraction of them reaches the extraction region.
A radially extending magnetic filter system installed in the source chamber divides the chamber into two axially separated regions: (1) the discharge or source chamber or region, where the plasma is formed and contains the energetic ionizing electrons, and (2) the extraction chamber or region where a plasma with colder electrons is found. The filter provides a limited region of transverse magnetic field, which is made strong enough to prevent the energetic electrons in the discharge chamber from crossing over into the extraction chamber.
U.S. Pat. Nos. 4,447,732 and 5,198,677 issued May 8, 1984 and Mar. 30, 1993 to Leung et al. show a multicusp ion source with a radially extending magnetic filter formed of two or more parallel magnets in a plane perpendicular to the beam axis.
A multicusp source equipped with a prior art magnetic filter can reduce the energy spread substantially. The axial plasma potential (V.sub.p) is different when the ion source is operated without and with a magnetic filter. Without the filter, V.sub.p decreases monotonically towards the plasma electrode. Positive ions formed on one side of the maximum can roll down and reach the extractor. Since the ions are generated at positions with different plasma potential, they will have a spread in axial energy when they arrive at the extraction aperture.
In the presence of a filter, the plasma potential distribution is very uniform in the discharge chamber region. Primary electrons emitted from a filament cathode are confined in the source chamber by the filter magnet fields as well as the multicusp fields on the chamber walls. Only very cold plasma electrons are present in the extraction chamber. The potential gradient in this region produces no effect on the axial ion energy spread. Since all the positive ions are produced within the source chamber region, they arrive at the plasma electrode with about the same energy due to the uniform V.sub.p distribution, or at most with energy spread given by the smaller potential drop between the center and the filter (.about.30 Gauss) region. One therefore expects that the longitudinal energy spread of the ions should be reduced.
Without the filter, the energy spread is found to be .about.5 eV. In the presence of the prior art radially extending filter, this energy spread is reduced to about 1 eV. However, the lowest energy spread that one can achieve should be approximately equal to the thermal energy of the ions, e.g. less than 0.1 eV for helium ions. Thus an improved magnetic field which produces axial energy spread &lt;1 eV is desired.